\r not escaped correctly in map literal

[jmesserly]

from package:matcher/src/util.dart:

/// A [Map] between whitespace characters and their escape sequences.
const _escapeMap = const {
  '\n': r'\n',
  '\r': r'\r',
  '\f': r'\f',
  '\b': r'\b',
  '\t': r'\t',
  '\v': r'\v',
  '\x7F': r'\x7F', // delete

generates approximately this...

  let _escapeMap = dart.const(dart.map({'\n': '\\n', '
': '\\r', '': '\\f', '\b': '\\b', '\t': '\\t', '\v': '\\v', '�': '\\x7F'}));

[jmesserly]

another one test case the same package (src/string_matchers.dart):

bool _isWhitespace(String ch) =>
    ch == ' ' || ch == '\n' || ch == '\r' || ch == '\t';

[jmesserly]

https://codereview.chromium.org/1183453005

[jmesserly]

\r and \f were both broken. \r was strangely omitted, \f had a case but wasn't matched by the RegExp